The invention pertains to a microphone amplifier having a digital control and more particularly to a two stage amplifier in which the first stage employs a feedback circuit providing relatively large gain control increments and a second stage employs an attenuator providing relatively small gain control increments so that the resulting overall gain control is substantially linear with equal increments.
Variable-gain microphone preamplifiers are employed in various applications including mixing consoles for sound reinforcement or recording, architectural electro-acoustic systems, public address (PA) systems and the like.
In large systems, the audio equipment is installed in a different area or location remote, from the control room. This makes remote control of the microphone amplifier gain highly desirable and necessary. However, satisfactory remote control is difficult to achieve.
It is desirable to digitally control the gain of a microphone amplifier (MA) in equal logarithmic increments, assuring at the same time good resolution and continuity. This is likewise difficult to achieve economically.
Typically the processing or analog to digital (A/D) conversion, if any, of an audio signal occurs at signal levels of about 1V or higher. A typical dynamic microphone has an output of about 1 mV. An amplifier gain of about 60 dB (.times.1000) is necessary to elevate the low microphone output signal to a level that can be either converted into digital form or further processed in the analog domain.
On the other hand, different microphone designs and wide dynamic range of the audio-source signal can cause the microphone output level to vary drastically, requiring the microphone amplifier gain to be made variable within the same wide range.
In a typical application, the control element is a potentiometer with a special taper located on the front panel of the piece of audio equipment. This makes remote gain control difficult if not impossible, leaving manual control as the only practical control option.
A microphone amplifier (MA) is typically designed as a linear (no distortion), high-gain (60 dB), variable gain (0.about.60 dB), high headroom (20 dB) amplifier with a relatively low equivalent input noise (EIN) level of less than -125 dBu at maximum gain. A well designed MA should also meet the following requirements:
1. Balanced input for high Common Mode Rejection Ratio (CMRR). PA1 2. Operative with low input levels 0.5.about.1 mV. (-60 dBu) and high output (about 0 dBu) suitable for further processing or A/D conversion. PA1 3. Low noise: SNR.about.70 dBu @ maximum gain of 60 dB and EIN&lt;-125 dBu. PA1 4. Wide gain range (0.about.60 dB) in virtually all cases accomplished by a potentiometer. To achieve continuity and uniform gain change in dB over the entire control range the potentiometer has a special, close to, but steeper than reverse-logarithmic (rev-audio) taper. PA1 5. Sufficient headroom of 20+dB above the nominal level of usually 0 dBu. PA1 6. Balanced output for better interface with a consequent piece of equipment or A/D) converters. PA1 7. Equal (uniform) logarithmic increments (dB). The incremental control comes from the nature of any digitally controlling or processing system, where all parameter changes can be implemented only in discrete steps. PA1 8. provide smooth, uniform gain control in a sufficient number of steps without an unreasonable increase in hardware. This is directly related to the practicality of the design.
The gain change should not affect the CMRR.
In addition, an MA desirably includes the option of:
A microphone amplifier with digital gain control should have the same electrical and noise characteristics as a conventional high quality microphone amplifier plus the added flexibility of digitally controlled gain in:
Finally, an MA should:
One conventional approach uses a digital volume control integrated circuit. These are so called digital potentiometers basically consisting of a resistive ladder and analog switches providing attenuation, or sometimes a gain, that works like a mechanical potentiometer with large number of tabs or steps. An associated digital circuit controls the transfer (attenuation) ratio and provides the interface with a CPU.
Some exemplary digital potentiometers are passive attenuators. Some are combined with operational amplifiers. Most can be connected in different feedback circuits to replace variable resistors and to perform a variety of gain control functions. Some provide linear or logarithmic change of the transfer ratio in 8 to 256 increments of 0.5 to 1 dB in wide range (80.about.126 dB). Used by themselves, however, digital potentiometers all are unsuitable for a microphone amplifiers, because they do not meet requirements 1, 3, 5 and 6.
In another approach, an analog microphone amplifier is used. This approach may meet some of the desired requirements, but does not meet requirements 4, 7, and 8.
A significant problem is that even if a suitable analog circuit for a microphone amplifier is selected in order to meet the balance, noise, and headroom requirements, implementing a uniform gain change of 60 dB in equal, logarithmic increments of 0.5 to 1 dB would require between 60 and 120 separate steps. Although having such large number of steps, existing digital potentiometers can not provide a uniform incremental gain change, because none meet the requirement 4.
The trade-off appears to be between good microphone amplifier performance with non-uniform gain change in equal increments in dB, or using an analog circuit combined with an integral digital potentiometer, providing uniform incremental gain change in dB, but mediocre or poor microphone amplifier performance.
Yet another approach uses an appropriate MA with a discrete resistive ladder and switches in the feedback loop. The way to achieve a uniform step in dB is to utilize an appropriate MA and to optimize the resistor values in the feedback resistive ladder. This can only be accomplished with a discrete design, which may solve the noise problem by using an appropriate MA. However such an implementation would require a prohibitively high number of switches and precise resistors, one each per step: 60.about.120. As a result, the discrete design is not practical and does not meet requirement 8.
Nevertheless, it is desirable to use discrete switches because the discrete approach offers a complete solution. Therefore an alternative solution using discrete switches is desirable if it can be made in a practical manner.